1. Field of the Invention
This invention relates to a developer for developing electric latent images or magnetic latent images in electrophotographic processes, electrostatic printing processes and the like, and a process for producing the developer.
2. Description of the Prior Art
Heretofore there have been known various electrophotographic processes such as those disclosed in U.S. Pat. No. 2,297,691, British Patent Nos. 1,165,406 and 1,165,405. These processes usually comprise utilizing a photoconductive material, forming electric latent images on a photosensitive member by some means, developing the latent images with a toner, if desired, transferring the developed toner images to a receiving sheet such as paper and then fixing the toner images by heat, pressure, or solvent vapor.
There are known various methods for visualizing electric latent images with a toner. For example, there may be mentioned magnetic brush development as described in U.S. Pat. No. 2,874,063, cascade development as described in U.S. Pat. No. 2,618,552, powder cloud development as described in U.S. Pat. No. 2,221,776, fur-brush development, liquid development and the like.
Developed toner images may be transferred to a receiving sheet and fixed, if desired.
As a method of fixing toner images, there may be mentioned a method comprising heating and melting toner particles by a heater or heat roller and fusing followed by solidifying on the support, a method for fixing toner particles to a supporting member by softening or melting the resin binder in toner particles with an organic solvent, a method for fixing toner particles to a supporting member by pressure, and the like.
Toners used for these development methods are heretofore fine powders composed of dyes or pigments dispersed in natural or synthetic resins. If desired, third components are added to the fine powders.
Materials for toner particles are selected in such a way that they are suitable for each particular fixing method. Therefore, a toner suitable for a particular fixing method is usually not usable for the other fixing methods.
In particular, a toner used for, conventional heat fusing fixing methods employing a heater is not likely to be used for a heat roller fixing method, a solvent fixing method, a pressure fixing method and the like. Therefore, toners suitable for each particular fixing method are researched and developed.
The method for fixing toners by pressure is for example, disclosed in U.S. Pat. No. 3,269,626. The method has various advantages, that is, less consumption of energy, non-pollution, copying without a waiting time by simply switching on a copying machine, no fear of burning and scorching copies, high speed fixing and a simple fixing device and the like.
However, the pressure fixing method has same disadvantages such as poor fixability of the toner, offsetting to the pressure roller, causing paper to wind on the pressure roller and the like. Therefore, researches have been made to improve the pressure fixation. For example, British Patent No. 1,210,665 discloses a pressure fixation toner containing an aliphatic component and a thermoplastic resin; U.S. Pat. Nos. 3,788,994 and 3,974,078, British Patent No. 1,431,699 and Japanese Patent Laid-open No. 108134/1977 disclose pressure fixable toners of a capsule type containing a soft material in the core; and British Patent No. 1,414,159 discloses a pressure fixable toner composed of a block copolymer derived from a sticky and strong polymer and a soft polymer.
However, any practically satisfactory pressure fixable toner has not been yet obtained which can be easily produced, has a sufficient pressure fixability, does not cause offsetting to the pressure roller, does not cause paper to wind on the pressure roller, is stable with repect to developing property and fixability upon repeated use, does not adhere to carrier, metal sleeve or the surface of the photosensitive member and has good storage stability, i.e. non-agglomerative and non-caking.
There has been recently used a method of developing electrostatic images with a one-component developer which has toner particles containing magnetic fine powder and does not use any carrier particles. In this method the toner binder resin is required to have good dispersibility and contacting property with respect to the magnetic fine powders and the toner particles are required to have high impact strength and fluidity.
One-component developers are proposed in U.S. Ser. No. 141,919 filed April 21, 1980 as a continuation application and U.S. Ser. No. 264,516 filed May 18, 1981 as a continuation application. When development is effected by triboelectric charging caused by friction between a one-component developer and a developing sleeve roller, the insulating material separates due to shock or a long time use, attaches to the sleeve roller by triboelectric action and accumulates, and thereby the developing durability is adversely affected. In this way, one-component developers suffer from various problems.
As shown in U.S. Pat. No. 3,775,326, Japanese Patent Publication No. 35867/1976, Japanese Patent Laid-Open No. 64931/1976, U.S. Pat. Nos. 4,022,738 and 4,108,653, it is known that polyethylene resin is a material having excellent pressure fixability. However, there are no known durable developers which reveal excellent pressure fixability even with a low pressure and can stand up to producing several ten thousand sheets of copy.
In pressure fixable toners, fixability contradicts developing durability. The larger the fixability, the poorer the flowability of toners, and thereby the developing durability becomes remarkably poor, and in an extreme case, after producing several copies only, any further reproduction becomes impossible. Toners capable of being fixed at a low pressure are desired for the purpose of avoiding curling of papers imparting, luster to papers and to accomodate changes in the thickness of paper.
In order to enhance the developing durability of pressure fixable toners, toner particles are mixed with a flowability improver. However, when fine particulates of a flowability improver having a size (about 5-50 m.mu.) far smaller than the particle size of the toner particle are used, the images produced at the beginning are good, but during repeated developing for copying, the fine particulates of the flowability improver become buried in the toner particles or separate from the toner particles, and as a result the flowability improver necessary for reproduction of images is liable to be lacking and therefore, the resulting image density is very low. Such undesirable phenomena seem to occur since the size of the flowability improver particulates is smaller than that of the toner and the hardness of the flowability improver particulates is harder than that of the toner. Such phenomena occur strongly when a pressure fixable toner capable of being fixed is used.